Current wireless communication systems utilize several different radio communication standards and operate at many different frequency bands. In this fractured service environment, terminals operating in multiple systems and frequency bands offer a better service coverage than single-band and single-system terminals. One example of a multiband communication terminal is a mobile phone operating for example at four GSM bands, namely GSM850 (824-894 MHz), GSM900 (880-960 MHz), GSM1800 (1710-1880 MHz), GSM1900 (1850-1990 MHz) and further at the UMTS band (1920-2170 MHz).
Compact multiband antenna configurations with good performance are needed to realize multiband mobile terminals and/or base stations. Current mobile terminals typically have one multiband antenna and one feed for the GSM bands and another antenna and feed for UMTS. At the same time as the space for the antennas in the mobile terminal is becoming very limited, there is a need to fit more and more antennas inside the terminal, for example to implement mobile antenna diversity.
Antenna diversity can be and is used to improve the performance of radio devices in a multipath propagation environment. In antenna diversity, two or more antennas operating at the same frequency band are used to receive the same information over independently fading radio channels. When the signal of one channel fades, the receiver can rely on the other antenna(s) to offer a higher signal level. Alternatively, it is also possible to combine two or more signals in such a way that interference caused by other transmitting devices reduces. The price for improved performance is, however, increased complexity. Generally, diversity can provide, for example, better call quality, improved data rates, and increased network capacity without the use of extra frequency spectrum. Diversity can also provide longer battery life or duration. When implemented in mobile terminals, the benefits of antenna diversity can be utilized without investments in the network infrastructure. In mobile terminals, the use of multiple antennas for one system can also reduce the effect of the user on the antenna performance.
However, some problems relate also to sizes of antennas. One of the main problems of small antennas is small operation bandwidth. The bandwidth is interrelated with efficiency and antenna size so that one of the mentioned characteristics can only be improved (antenna size decreased) at the expense of others. For example, if a larger antenna bandwidth is needed for a new communication system or for implementing a new antenna function, such as diversity, the simplest way to do this is to increase the antenna size or to trade off some of the total efficiency. However, in small portable radio equipment neither one of the mentioned methods is desirable. Usually, they are accepted only in compelling circumstances.
Fortunately, there are known methods, such as, introducing multiple resonances with resonant matching circuits and parasitic elements, which can be used to increase the operation bandwidth up to a certain limit without degrading the efficiency. However, these methods typically increase the complexity of the antenna.
Furthermore, the performance of a small antenna in a relatively small terminal depends also on the location and orientation of the antenna(s) as well as the size and shape of the terminal. Finding suitable locations for the antenna in the terminal can be at least as important for the performance as the actual antenna structure.
To design compact and efficient internal handset antennas that operate e.g. at four GSM bands (GSM850/900/1800/1900) and the UMTS band is very challenging. The problem becomes even more difficult, if multiple antennas operating at a given frequency band, such as diversity antennas, have to be included in a mobile terminal. If total antenna size cannot be increased when a new antenna or operation band is added, the sizes of the existing antennas must then be decreased, which leads without exception to degradation of the performances of the existing antennas.
Further problems arise when two antennas operating at the same frequency range are placed close to each other because they tend to couple to each other. In diversity and MIMO (Multiple Input Multiple Output) applications, mutual coupling decreases the efficiency of the coupled antennas reducing the improvement from that, which would be possible with perfectly isolated antennas, which can be designed more independently. Furthermore, mutual coupling complicates antenna design as it causes modifications made to one antenna to affect also the others. Large isolation between antennas operating at different bands is also useful because it can allow simplifying the RF front end. However, it can be very challenging to design antennas that have e.g. over 10 dB isolation in the limited space allowed for internal antennas of modern mobile terminals.
In addition low correlation between antenna signals is a prerequisite for the improvement of the radio link performance with diversity or MIMO. Generally, it is not obvious that low correlation can be achieved in the small space allowed for the internal antennas of modern mobile terminals. In current mobile phones, various components such as a camera, speaker or both have often been located at least partly between the internal antenna element and its ground plane. These additional components can degrade the antenna performance. Thus it would be desirable to find antenna solutions that would enable the integration of other components in their proximity with minimal degradation of antenna performance.
Furthermore, the hand of a user can also be problematic for antenna performance, because it typically degrades the performance of mobile phone antennas at the frequency ranges in question (0.8 GHz-2.2 GHz). The effect is very strong when the hand is at least partly on top of the antenna, and unfortunately it is very common that the user often holds the phone so that the forefinger is on top of the antenna element near the top of the phone. It would be desirable to find antenna configurations in which internal antennas are placed so that the effect of the user's head, hand, or other body parts have a minimal influence on their performance.
Relating to the problems mentioned above, it is known that the bandwidth of a small antenna can be increased using resonant matching circuits and parasitic elements. Moreover, it is known that generally increasing the distance between antennas increases isolation between them. In addition, it is well accepted that the isolation depends on the relative orientation of the antennas.
However, according to the inventors' experience, when additional antennas are added between two antennas to be isolated, whether the isolation increases or decreases depends on the type of the additional antenna as well as its relative orientation to the antennas to be isolated. Hence, it is not obvious that merely locating any antenna or resonator between two antennas automatically increases isolation between them; it may also do the opposite.